(1) Field of the Invention
The present invention relates to an optical waveguide reduction optical image sensor using optical waveguides, which is used for a one-dimensional read optical system for producing hard copy and the like and is applied to a facsimile, a copying machine, an image scanner, and the like.
(2) Description of the Prior Art
With the recent increase in image readers such as facsimiles, image scanners, and digital copying machines, demands have arisen for a high-performance, compact one-dimensional image sensor for converting image information into an electrical signal. The conventional one-dimensional image sensors include a reduction optical image sensor for reading a focused image by using a reduction optical system such as lenses, and a contact type image sensor for reading a one-to-one focused image by using a one-to-one optical system.
The reduction optical image sensor is inexpensive and capable of high-speed read. However, since this sensor must perform a reduction imaging operation by using lenses, the apparatus size becomes large, resulting in difficulty in reducing the size. In addition, complicated adjustment is required for the optical system of each apparatus.
In contrast to this, the contact type image sensor does not require complicated adjustment for the optical system because the distance from an original to the photoelectric conversion element array is short. However, the size of the photoelectric conversion element array is large, and a complicated electronic circuit is required to drive the photoelectric conversion element array. For this reason, it is difficult to attain a reduction in cost.
Under the circumstances, a reduction image sensor using an optical waveguide array has recently been proposed. This sensor is constituted by a microlens array having a width almost equal to the width of an original surface, an optical waveguide array constituted by a plurality of waveguide cores used to guide light focused by the microlens array and formed in a cladding substrate, and a photoelectric conversion element array for receiving light and converting it into an electrical signal. According to this reduction image sensor using the optical waveguide array, reductions in the cost and size of the apparatus can be realized, and complicated adjustment for the optical system is not required.
Several methods of manufacturing the optical waveguides of such a reduction image sensor using a polymeric material for cores are known.
One method is a sandwich method. According to this method, a polymer precursor material for the cores of waveguides is filled in the groove portions of a pattern substrate made of a polymeric material such as PMMA and having groove patterns, and a flat substrate made of a polymer such as PMMA is brought into tight contact with the groove portions of the pattern substrate. Thereafter, photopolymerization is caused by ultraviolet radiation to form the cores of the optical waveguides made of the polymeric material.
Another method is a vacuum capillary method. According to this method, the pattern surface of a pattern substrate on which groove patterns serving as capillaries are formed is brought into tight contact with a flat substrate to form capillaries. Thereafter, a polymer precursor as a material for the cores of optical waveguides is filled in the capillaries by capillarity, and the polymer precursor is polymerized.
According to these methods, since there is no gap between the pattern substrate and the flat substrate, polymeric optical waveguides having excellent optical waveguide characteristics can be realized, which are free from crosstalk caused by light leaking from the gaps between the cores.
In the reduction optical image sensor using the polymeric optical waveguide array, an original surface is illuminated by an LED in an image read operation, and information on the original surface is guided to the photoelectric conversion elements through the core portions of the optical waveguides, thereby reading image information.
In the above conventional optical waveguide reduction optical image sensor using a polymeric material, if the coefficient of linear thermal expansion of a polymeric material such as PMMA for the polymeric optical waveguide array constituted by a plurality of cores differs from that of the photoelectric conversion element array, a pitch offset occurs between the polymeric optical waveguide array and the photoelectric conversion element array at the joint portion therebetween with a change in temperature. For this reason, a pixel offset occurs, and an output from each photoelectric conversion element deteriorates. An image cannot therefore be properly read.
When such polymeric optical waveguides are to be used, therefore, it is very important to prevent a decrease in image read precision due to a change in temperature.